Converged networks, in which data and voice are transmitted over the same network, are becoming increasingly popular. Transmitting voice via a data network offers several advantages over the use of a traditional voice network. For example, long-distance calls over a traditional voice network generally have a higher cost than long-distance calls over a data network. Because voice signals are transformed to data during a telephone call over a data network, calls made over a wide area data network (WAN) generally cost no more than calls made over a local area network (LAN), as the cost of sending data across a data network is generally not a function of the distance between the communicating parties. Thus, long-distance calls over a data network cost no more than local calls.
The equipment necessary to enable voice transmission over a data network generally includes a gateway, which is a computer used to interface an analog telephone line to the data network. The gateway converts voice signals to and from the correct format for the data network protocols, and also may compress the voice data, demodulate fax signals, etc. Many different network protocols may be used to send voice data. Because of the widespread implementation of versions of the internet protocol (IP) over both LANs and WANs, IP is becoming the most common network protocol for voice transmission.
With traditional voice networks, each telephone call takes place over a dedicated circuit that is established for the call and maintained open until the call is terminated. Because of this, the amount of network resources used by a single telephone call is generally proportional to the duration of the telephone call and the physical distance separating the ends of the call. In contrast, because no dedicated circuit is kept open during a telephone call over a data network, the amount of network resources used by the telephone call is generally proportional to the amount of data sent during the call (and increased if a priority scheme is used to increase the transmission rate of the telephone call).
To decrease the bandwidth used by a telephone call over a data network, many gateways utilize silence suppression technology. Silence suppression technology recognizes periods of silence in a conversation, and stops the transmission of data packets across the network during these periods. Silence suppression effectively allows two people to talk in almost half the bandwidth that would be needed for a conversation in a true full-duplex communication mode. However, silence suppression also causes the line to sound completely quiet or dead to a listener on the receiving end, which can be disconcerting to the listener.
To compensate for the dead line sound caused by silence suppression, “comfort noise” is sometimes generated to let a listener know that the communication path is still open. Comfort noise is artificial white noise that sounds like background noise heard during a voice-network telephone call, and makes a telephone call over a data network sound more like a familiar voice-network telephone call.
Because comfort noise makes a telephone call held over a data network sound more like an ordinary voice network telephone call, it may make data-network calls more appealing to users. However, current methods of generating comfort noise also may have drawbacks. First, when comfort noise is generated by transmitting-side equipment, a continuous stream of packets of noise data are still being sent across the network. This wastes network bandwidth, and thus somewhat negates the advantages of using silence suppression. Second, when comfort noise is generated by receiving-side equipment, problems may arise with holdover time (the time between the beginning of a conversational pause and the beginning of the generation of comfort noise) and front-end clipping (the cutting of the first sounds of words) due to possible errors in recognizing the beginnings and ends of words or conversational phrases.